The present invention relates to the art of induction heating and, more particularly, to an improved induction heating furnace which includes a control device for maintaining predetermined temperatures of heated workpieces in a multiple induction coil furnace.
The present invention is particularly applicable to inductively heating elongated or effectively continuous bar stock or elements to a processing temperature for subsequent forging, forming or treating; however, it should be appreciated that the invention has much broader applications and may be used for inductively heating various metal workpieces over a wide range of frequencies for various processing functions.
In metal stock treatment installations, consistency in the characteristics of completed articles is often required. Uniformity of the characteristics of forgings is particularly important with regard to the degree of quality and overall strength of a particular forging. Quality and strength of the eventual forged item are directly related to the temperature of the stock used for forging. Thus, in order to obtain identical characteristics of forged workpieces, the heating of the stock or billet, prior to forging, must be controlled to provide equal heating throughout the billet. A single induction heating furnace may be controlled to sequentially heat a number of billets to identical specifications. However, when a multiple induction heating furnace is required in order to heat more than one elongated bar, wherein a number of induction heating coils are arranged in electric parallel connection with a single power source, maintaining heating specifications for the bars is difficult. By providing induction heating coils of identical specifications, the parallel connection of the coils to a single power source necessitates equal power being applied at each induction coil. The application of equal power to a number of induction coils having similar characteristics would normally provide equal heating of workpieces extending through the induction coils. However, a number of further factors, variable for each of the parallel induction coils, effect the heating of a workpiece extending through the induction coils. These factors include the position of the workpiece within the coil, environmental conditions in the immediate vicinity of each coil, and most notably the slightly variable masses of the workpieces due to manufacturing tolerances.
The most common method of varying heating by individual induction coils of a multiple parallel arrangement at present involves the use of multiple tap induction coils. When the temperature of a workpiece is desired to be changed, one power source lead at the coil is disconnected and reconnected at a different tap along the coil. To increase the temperature of the workpiece the reconnection is made at a tap further along the coil, thus lengthening the coil. The temperature is decreased by reconnection of the lead at a tap which results in shortening the coil. Obviously, the use of multiple tap coils does not achieve infinite adjustability of the temperature within a coil. A slide variable tap coil is not practical as a result of the high currents required to be transferred to the coil for induction heating. Moreover, the operator intervention necessary to use multiple tap induction coils in disconnecting and reconnecting leads renders the process cumbersome at best.
Another technique used to equalize temperatures of multiple workpieces in parallel induction furnaces involves altering the speed at which one workpiece travels through the respective coil. While such an approach is acceptable in some situations, subsequent processing steps which require regulated delivery of the workpieces render speed control worthless.